An image sensor may be a semiconductor device that converts an optical image to an electrical signal. An image sensor may be classified as a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) image sensor (CIS). In a CCD, respective MOS capacitors may be arranged close to one another and charge carriers may be stored in the capacitors and transferred.
CMOS technology may use a control circuit and a signal processing circuit as peripheral circuits. A CMOS image sensor may include a number of MOS transistors corresponding to a number of pixels, and may detect outputs in sequence using the MOS transistors in a switching manner. A CMOS image sensor may include a photodiode and a MOS transistor in a unit pixel. A CMOS image sensor may sequentially detect signals in a switching manner, and may thus determine an image. Sensitivity of a photodiode may be important to an operation of a CMOS image sensor.